The present invention relates to a colour cathode ray tube having an in-line electron gun.
Electron guns for colour cathode ray tubes are arranged to generate three electron beams whose paths of propagation lie in a plane which is generally horizontal. The electron guns may be constructed so that there is one discrete electron gun for each beam or so that they have a number of electrodes in common, a so-called integrated electron gun structure. Integrated electron gun structures are inherently more compact and in consequence are popular for use in those colour cathode ray tubes, such as narrow-necked and mini-necked colour cathode ray tubes in which space is a premium. When designing and constructing an electron gun for a colour cathode ray tube various types of errors have to be taken into account and an optimum comprimise has to be decided upon in order to minimize the errors. The types of errors which are of interest are core haze eccentricity (CHE), beam displacement (BD) and free fall error (FFE). Core haze eccentricity occurs when the haze which surrounds the spot proper at the screen is located eccentrically with reference to the centre of the spot. Beam displacement occurs in respect of relative positions of the outer electron beams to the center electron beam. Free fall error (FFE) is effectively the convergence error at the screen. FFE can be corrected by altering the pitches of the outer apertures with respect to the central aperture in the electrodes of the triode part of the electron gun to obtain a desired angle of trajectory. However this also has an effect on CHE and BD. CHE can be reduced by ensuring that the converging electron beams pass through the centers of their respective focusing lenses. In simple terms these errors can be grouped in two classes namely focusing errors and convergence errors. Furthermore unless special precautions are taken, measures to reduce the effects of one type of error make the other type of error worse.
British Patent Specification No. 2031221 A (PHN 9215) discloses an in-line electron gun assembly in which focusing and convergence are independently adjustable. In the embodiments of the electron guns disclosed the convergence of the outer electron beams takes place in the prefocusing part of the electron gun and the electron beam focusing is carried-out using a bipotential electron lens. An embodiment of an integrated electron gun assembly shown in FIG. 4 of Specification 2031221A has three in-line arranged cathodes, a first grid, a second grid, a prefocusing grid, a focusing electrode an an accelerating electrode, all the grids/electrodes being orthogonal to the central longitudinal axis of the electron gun. Each grid/electrode has three in-line apertures of which the central ones are co-axial about said central longitudinal axis. However in order to obtain the required degrees of freedom the outer apertures in the prefocusing grid, the focusing electrode and the accelerating electrode are not only of differing sizes but their pitches, that is the distance from their centers to the central longitudinal axis, are different. Consequently no two grids/electrodes are the same.
Specification U.S. Pat. No. 4612474 discloses an in-line integrated electron gun having mirrored main focusing and accelerating electrodes. A pre-focusing electrode is provided between the triode (or beam forming) section of the electron gun and the main focusing lens. The outer apertures of the electrodes of the triode section are concentric about respective axes. The axes of the outer apertures in the pre-focusing electrode are displaced outwards relative to the first mentioned axes. Lastly the axes of the apertures in the main lens electrodes are displaced inwards relative to the first mentioned axes. By offsetting the axes in this way the outer electron beams are converged by the prefocusing lens. Such an arrangement provides two degrees of freedom, namely the eccentricity of the outer apertures in the pre-focusing electrode and the offsetting of the respective axes for optimising the spot error, beam displacement and beam asymmetry. Hence a compromise has to be made.
Another aspect to be considered is the assembly of the electrodes comprising the electron gun. Normally a jig is used having three substantially parallel insertion pins. Each pin has a plurality of steps of different cross-sectional area thereon which steps act as abutments for the mutual spacing of some of the electrodes in the axial direction, the mutual spacing of others of the electrodes being obtained by the use of spacers. Offsetting the axes of outer apertures in one or more electrodes requires the pins to be specially formed. This is both troublesome because the pins have to be specially formed and this constitutes an additional cost item because each type of electron gun requires its own jig.